1. The Field of the Invention
The present invention is directed to methods for preparing devices having hollow microchannels for use in various applications.
2. The Relevant Technology
Microchannel-containing devices have applications in various fields ranging from medical instrumentation to basic research in life sciences, earth sciences, and engineering. Hollow microchannels permit controlled dispensing of very small, i.e., microscale, quantities of fluid, such as, for example, in ink jet printers, or for virtually any type of preparation and/or analysis processing of microscale sample quantities. For purposes of this application, the term "microchannel" refers to enclosed or partially enclosed channels having heights within the range from about 2 to about 200 micrometers (.mu.m) and widths within the range of about 5 micrometers to about 2 millimeters. Exemplary micro analysis systems developed within the biotechnology field include gas chromatography, liquid chromatography, free-flow fractionation, electrophoresis, and polymerase chain reaction which require miniaturized channels for reactant delivery and biochemical reaction chambers.
Presently known techniques for fabricating hollow microchannels include etching or machining methods which form channels in the substrate surface. For example, narrow channels may be etched, by various known methods, into the surface of a substrate such as silicon, glass, or quartz and a cover plate, typically made of glass, is then bonded to the substrate surface such that the cover plate forms one side of the etched microchannels. These microchannels are typically up to about 50 micrometers in depth. Alternatively, narrow channel "halves" may be machined into two plastic substrates which are then aligned with each other and bonded together such that hollow microchannels are formed between the two bonded plastic substrates. This method can form hollow microchannels having circular profiles and diameters within the range of about 0.5 mm to about 1.6 mm.
For many microchannel applications, the addition of electronic circuitry connected to the microchannels or to the microchannel-containing device for various purposes is desirable. Electrodes can be used to resistively heat fluids inside hollow microchannels for processes, e.g., many biochemical reactions such as polymerase chain reactions, which require elevated temperatures. Electrical currents can also be used to effect field flows resulting in desired movement of fluids, or specific portions or components of fluids, within the microchannels, e.g., as in Electrical Field-Flow Fractionation separation systems for effecting biological cell separations based on molecular size differences. Signal processing circuitry is also valuable in many systems. Thus, it will be appreciated that it is often desirable to be able to incorporate integrated circuits into a microchannel-containing device.
The above-described methods suffer from the drawback of being incompatible with standard integrated circuit fabrication techniques. Because the planarity of the substrate surface is disrupted by the channel etching or machining process, neither of the above-described methods are compatible with a subsequent integrated circuit fabrication process. Thus, these microchannel forming methods are not compatible with subsequent incorporation of integrated circuitry.
There are some methods of forming hollow microchannels close to, or on, the surface of planar substrates such that electrodes or sensors may be subsequently integrated. One such method involves undercutting an etch mask during a wet etching process on a silicon or quartz substrate. The resulting partially closed channel is sealed with a silicon nitride or other suitable material deposited by a low pressure chemical vapor deposition (LPCVD) process. The depth of these channels may range from about 20 to about 100 micrometers. Another method uses a suitable deposition process, such as LPCVD, to deposit sacrificial phosphosilicate-glass (PSG) material upon a substrate surface to define the inner dimensions of a hollow microchannel being formed. A silicon nitride, silicon dioxide, or other suitable dielectric is deposited by LPCVD or other deposition process over the PSG, which is then removed using hydrofluoric acid. These microchannels are typically about 1-2 micrometers in both width and height.
While the just described methods of forming microchannels are compatible with subsequent integrated circuit fabrication, these methods have other drawbacks. The LPCVD process requires elevated temperatures which may be detrimental to other materials desired to be used in the microchannel-containing device. In addition, at least one wall of the hollow microchannel is formed by the deposited dielectric material. It is not possible to form this material into a relatively thick or a wide wall. Thus, the inner width and inner height of the hollow microchannel itself, as well as the thickness of the microchannel wall(s), is limited by these material limitations. The limited channel width and height results in problems in the flow dynamics of the microchannel since more pressure is required to move fluid through a narrower, rather than a wider, channel. The limited wall thickness decreases the strength and durability of the microchannel in general and is particularly a problem in view of the potentially high fluid pressures due to the limited widths and heights of the channels.
None of the above-described methods provide hollow metal microchannels. Metal microchannels have advantages over other materials for many applications. For example, metal or metal coated microchannels exhibit excellent strength and durability and certain metals are compatible with many process conditions including biochemical reactions. Currently, there is no method available for preparing hollow metallic microchannels on the surface of planar substrates.
In view of the above, it would be an advance in the art to provide methods of preparing devices having hollow metallic microchannels on a surface of a planar substrate. It would be a further advance to provide methods of preparing devices having surface metallic microchannels which do not require elevated temperatures and are compatible with standard integrated circuit fabrication techniques. In addition, methods of preparing devices having durable, thick-walled surface metallic microchannels with relatively large cross sections that provide efficient flow characteristics would be an advancement in the art.